1. Field
This invention relates to a method for joining plastic materials. More particularly, it relates to the bonding of materials by passing electromagnetic radiation through one or more transmitting plastic materials and into a radiation absorbing material with the generation of heat and resulting fusion of components to simultaneously form both lap and butt joints.
2. Background
Although one piece items are preferred because of their strength and exclusion of assembly operations, mechanical limitations and other considerations often make it necessary to join plastic materials to each other or to other parts. As a consequence, a number of methods for joining plastic materials have been developed. The use of mechanical fasteners is labor intensive, the plastic is corrupted by fastener holes resulting in mechanical joints that often leak, and the joints are weak. Press and snap fits are also used but this is often unsatisfactory since high stress components are unacceptable when the materials are subject to thermal cycling or to harsh environments.
In thermal welding, one or more of the plastic parts to be joined are placed close to or in contact with a heat source such as a hot plate or other appropriately shaped hot element device to melt the plastic at which time the parts are then pressed together to form the welded joint. Induction welding uses a material that is susceptible to heating in an induction field that is implanted within the thermoplastic matrix. Application of an induction field causes the implanted material to heat and melt the surrounding plastic material which, if in contact with the part to be joined, forms the requisite joint.
In vibration welding, one of the parts is held stationary and the other is vibrated to produce frictional heat. Alignment is critical and bonds may not be as strong as expected. In spin or friction welding, one part is held stationary while the other is rotated under sufficient pressure to keep the parts in contact with each other. The heat melts the surface at which point pressure is applied to complete the bonding process. In both vibration and spin welding, high forces are needed to generate the frictional heat to melt polymers like polyethylene and, as such, heavy and costly equipment is needed to provide substantial clamping force.
Solvents and adhesives have also been used to join thermoplastic materials. However, some solvents can adversely effect some plastics. Also, solvents present potential hazards to assembly personnel. Plastics can be bonded with a variety of adhesives but often these are solvent based and present the solvent problems noted above. In addition, adhesives can complicate polymer recycling. Both solvent and adhesive methods are complicated by waste and chemical disposal problems in addition to surface preparation requirements. Solvent and adhesive methods are typically used with polyvinyl chloride (PVC) piping.
Infrared lamps and laser beams have been used to bond one plastic to another but such techniques have been limited to single joint applications. Laser welding has additional problems. Bond strengths can be disappointing. Vaporization and flashing (ignition) of substrate can occur when gaps occur between the interface of the two materials. Laser welding also tends to create surface pits and craters. Although lasers have been on the market for a considerable time, their cost is still relatively high.
None of these methods have proven satisfactory for the joining of tubular parts such as plastic pipe. Although the following two methods of joining plastic pipe are widely used, nether of them is entirely satisfactory. In butt welding, the ends of the pipes are joined by bringing the ends of the pipes close to or in contact with a hot plate. After sufficient melting has occurred, the hot plate is removed and the ends pressed together and cooled under pressure. A good butt weld does not leave any gaps in the interface between the two pipes that are joined. Exterior weld defects can be readily determined by visual inspection and, if necessary, the joint rejected and rejoined. However, it is to be noted that a flash or weld bead is formed on both the interior and exterior of the pipe joint. The interior weld bead is disruptive of smooth flow within the pipe and leaves an uneven surface on the exterior of the pipe. Butt welds are typically used with straight runs of pipe that can be well supported to prevent failure of the joint. Butt welds are not typically used with small pipe fittings as the bulk heating can result in considerable distortion of the fittings, especially when thin-wall fittings and pipe are used.
In lap welding, an additional fitting, referred to as a coupling or collar, is slipped over the end portions of the pipes and joined to the exterior pipe surface by melting the interior surface of the coupling and/or the exterior surface of the pipe or by using a solvent or adhesive. Heating is the common method of pipe lap joint welding with adhesives or solvents being using with only a small amount of piping such as PVC pipe. Although hot element type heating can be used, implanted resistive element heating is preferred as it allows better control of the melting process. Lap joint tends to be strong because the fitting overlaps the end portions of both pipes and considerable surface area on both pipes is involved in the joint. However, the joint cannot be visually inspected and failure to evenly apply solvent, adhesive, or heat to the coupling and/or radial surface portions of the pipe can leave undetected weak joints. The ability to visually inspect lap joints has been a long felt need in all industries using plastic pipe. Visual observation would reveal the amount of fusion of the collar and pipe in the joint and thereby provide a strong indication of the soundness of the joint as well as revealing defects such as bubbles caused by overheating and decomposition of the plastic pipe or the failure to adequately melt the plastic. Another problem with the lap joint is that it leaves an open interface, i.e., gaps, between the ends of the joined pipes. Such gaps are regions of potential accumulation of microbes and foreign debris that can be quite detrimental in industries using plastic pipe for transportation of fluids such as high purity water and other liquids including semiconductor, food, pharmaceutical and chemical solutions. The use of resistive heating elements imbedded in the plastic matrix of the joint components further accentuates the contamination problem when the element melts through the plastic during the joining process and later comes in contact with the passing fluid.
It is therefore, an object of this invention to make a doubly secure and strong joint
It is on object of this invention to provide a joint with no gaps or defects in the butt weld interface.
It is on object of this invention to provide a joint with no gaps or defects in the lap joint weld interface.
It is an object of this invention to provide a butt joint without a weld bead on the interior of a pipe.
It is another object of this invention to make simultaneously both a butt and lap joint.
It is another object of this invention to provide for visual inspection of a lap joint.
It is an object of the present invention to heat only those portions of a joint involved in bonding while leaving other portions of the materials essentially unaffected, undistorted, and in their initial (pre-bonding) configuration.
It is a further object of the present invention to avoid the use of solvents and other environmentally unfriendly bonding adhesives.
It is an object of the present invention to avoid contamination from electrically resistive heating elements.
It is an object of the present invention to provide a uniform and strong bond along the bond line.
Other objects of the invention will become apparent to those with ordinary skill in the art from consideration of the present disclosure.
To meet these objects, a joining method of the present invention features a method for the simultaneous formation of both butt and lap joints. The method comprises the steps of:
1) Providing a radiation source of electromagnetic radiation in the form of poly-chromatic, non-coherent radiation such as from a quartz-halogen lamp or stimulated, monochromatic, coherent radiation from a laser source.
2) Providing work pieces in a butt joint configuration, that is, the end of a first piece is butted against the end of a second piece to form a butt joint relation defining a first interface.
3) Placing a sleeve over the end portions of the first and second pieces to form lap joint configurations. The end portion of the first piece forms a lap joint relation with the sleeve and defines a second interface. The end portion of the second piece forms a second lap joint with the sleeve and defines a third interface. The edge of the butt joint is sandwiched between the first and second lap joints.
4) Providing at least a portion of one of the pieces as a substantially rigid radiation transmitting material, that is, the end portion of the first piece, the end portion of the second piece, or the lap joint piece (sleeve) must be a radiation transmitting material.
5) Providing an absorbing material that absorbs the electromagnetic radiation with the generation of heat. The absorbing material is placed in at least a portion of a bond line formed by the first interface, the second interface, and the third interface. However, it is possible to omit the absorbing material from one of the interfaces and rely on conductive heating of that interface. The quantity of heat produced by the absorption of the radiation must be sufficient to bond: a) the ends of the first and second pieces with a butt joint, b) the end portion of the first piece to the sleeve with a lap joint, and c) the end portion of the second piece to the sleeve with a second lap joint.
6) After the above arrangement of the first and second work pieces and the sleeve have been made along with the proper selection and arrangement of transmitting and absorbing materials, the electromagnetic radiation from a radiation source is passed initially through a transmitting plastic and then to the absorbing material. It is noted that a wide variety of combinations and arrangements of transmitting and absorbing materials can be made. For example, the absorbing material can be provided as an integral part of one of the components, that is, the first piece, the second piece, or the sleeve. The absorbing material can be provided as a separate component that is placed in one or more of the three interfaces. The absorbing material can be selected from a wide range of materials with carbon black having been found to provide good radiation absorption even in small quantities of less than a fraction of a percent. Emulsions of carbon black and other colored pigments that can be readily painted onto the interface materials have been shown to be a good source of absorbing material. The absorbing material can be combined with a filler material, preferably a material similar in composition to the work pieces or the sleeve. In such a case, the absorbing and filler material combination can be a thin film of black plastic such as found in electrical-type tapes or thin film sheet material. If transmitting materials are selected that are sufficiently clear, visual inspection of the resulting joints can be made after the joining process is complete.
7) Sufficient electromagnetic radiation is then absorbed by the absorbing material to produce enough heat in the bond line to provide sufficient molten material to afford fusion of the ends and end portions of the first and second pieces and the lap joint sleeve.
8) The bond line is then cooled to bond the ends of the first and second piece in a butt joint, the end portions of the first and second pieces and the sleeve in lap joints.
The method is particularly effective for joining plastic pipe with simultaneous butt and lap joints. One of the features of the invention is the omission of absorbing material in the interface between the ends of the pipe forming the butt joint. By relying on conductive heating from absorbing material in the lap joint interfaces, it is possible to obtain a good butt joint between the ends of the pipe. In addition, the joint is smooth and gap free about the internal pipe bore. This has the advantage of eliminating the possibility of absorbing materials contaminating the fluids flowing in the pipe and also eliminating any gaps where debris and living organisms may accumulate. Such a feature is especially important in those industries where the purity of fluids flowing within in the pipes must be maintained at high levels.
For pipe joints, the coupling sleeve can be provided on the outside of the pipes or placed within the bore of the pipes. When on the exterior of the pipes, a sleeve may make it difficult to pass the pipe through small openings. When on the interior of the pipe, the sleeve partially obstructs fluid flow and renders the flow more turbulent. To avoid such difficulties, a recess can be formed in the end portion of each of the pipes to accept and accommodate the sleeve. A recessed circular surface and a radial ledge define the recess in the end portion of each pipe. When the ends of the pipe are placed in butt relation, the two recesses accept the coupling in mating relation.
The ends of the coupling form a butt joint relation with each of the radial ledges in the end portions of the pipe. These butt joint relations give rise to a fourth and fifth interface of the bond line. Absorbing material is typically provided in these interfaces. The recesses may be formed on either the exterior or interior of the pipe. When the recesses are on the interior of the pipe, a coupling filled with absorbing material eliminates the need for applying absorbing material to the various interfaces and provides a convenient, low cost source of absorbing material.
The foregoing and other objects, features and advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail and illustrated in the accompanying drawings. It is contemplated that variations in procedures, structural features and arrangement of components may appear to those skilled in the art without departing from the scope of or sacrificing any of the advantages of the invention.